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International audience; The photoluminescence (PL) of monolayer tungsten disulfide (WS2) is locally and electrically controlled using the nonplasmonic tip and tunneling current of a scanning tunneling microscope (STM). The spatial and spectral distribution of the emitted light is determined using an optical microscope. When the STM tip is engaged, short-range PL quenching due to near-field electromagnetic effects is present, independent of the sign and value of the bias voltage applied to the tip–sample tunneling junction. In addition, a bias-voltage-dependent long-range PL quenching is measured when the sample is positively biased. We explain these observations by considering the native n-doping of monolayer WS2 and the charge carrier density gradients induced by electron tunneling in micrometer-scale areas around the tip position. The combination of wide-field PL microscopy and charge carrier injection using an STM opens up new ways to explore the interplay between excitons and charge carriers in two-dimensional semiconductors.

International audience; 2D materials beyond graphene and in particular 2D semiconductors haveraised interest due to their unprecedented electronic properties, such as highcarrier mobility or tunable bandgap. Blue phosphorene is an allotrope of blackphosphorene that resembles graphene as it presents a honeycomb structure.However, it is known to have semiconductor character and the crucial pointis to determine whether this hexagonal phase of phosphorene presents Diracfermions as in graphene. Here, the first compelling experimental evidenceof Dirac fermions in blue phosphorene layer grown on Cu(111) surface ispresented. The results highlight the formation of a highly ordered bluephosphorene sheet with a clear Dirac cone at the high symmetry points of theBrillouin Zone. The charge carriers behave as massless relativistic particles.Therefore, all the expectations held for graphene, such as high-speed electronicdevices based on ballistic transport at room temperature, may also beapplied to blue phosphorene.

Blue phosphorene (blue-P) has attracted considerable attention due to its potential applications in optical and electronic devices. However, its synthesis has remained a challenge. Here, we report an experimental investigation of the first steps of blue-P growth on Au(1 1 1) surface by molecular-beam epitaxy. The structure was characterized by in situ low temperature scanning tunneling microscopy, low-energy electron diffraction, combined with density functional theory calculations. We reveal two-dimensional (2D) phosphorus clusters (P-clusters) formed on surface at 150 °C, where the most prevalent structure of P-clusters is composed of triangles with six protrusions. We also demonstrate the transformation of these P-clusters into a single layer of blue-P after post-annealing at 260 °C. Our observation of the growth process is a necessary step for exploring the growth mechanisms further.

This work reports on the electron-induced modification of NaCl thin film grown on Ag(110). We show using low energy electron diffraction that electron beam bombardment leads to desorption and formation of Cl vacancy defects on NaCl surface. The topographic structure of these defects is studied using scanning tunneling microscopy (STM) showing the Cl defects as depressions on the NaCl surface. Most of the observed defects are mono-atomic vacancies and are located on flat NaCl terraces. Auger electron spectroscopy confirms the effect of electron exposure on NaCl thin films showing Cl atoms desorption from the surface. Using density functional theory taken into account the van der Waals dispersion interactions, we confirm the observed experimental STM measurements with STM simulation. Furthermore, comparing the adsorption of defect free NaCl and defective NaCl monolayer on Ag(110) surfaces, we found an increase of the adhesion energy and the charge transfer between the NaCl film and the substrate due to the Cl vacancy. In details, the adhesion energy increases between the NaCl film and the metallic Ag substrate from 30.4 meV Å−2 for the NaCl film without Cl vacancy and from 39.5 meV Å−2 for NaCl film with a single Cl vacancy. The charge transfer from the NaCl film to the Ag substrate is enhanced when the vacancy is created, from 0.63e− to 1.25e−.

The emergence of peculiar phenomena in 1D phosphorene chains (P chains) has been proposed in theoretical studies, notably the Stark and Seebeck effects, room temperature magnetism, and topological phase transitions. Attempts so far to fabricate P chains, using the top-down approach starting from a few layers of bulk black phosphorus, have failed to produce reliably precise control of P chains. We show that molecular beam epitaxy gives a controllable bottom-up approach to grow atomically thin, crystalline 1D flat P chains on a Ag(111) substrate. Scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density functional theory calculations reveal that the armchair-shaped chains are semiconducting with an intrinsic 1.80 ± 0.20 eV band gap. This could make these P chains an ideal material for opto-electronic devices., Similarly to graphene, attempts to fabricate phosphorene by epitaxy or starting from a few layers of bulk black phosphorus have failed so far. Here, the authors present a controllable bottom-up approach to grow atomically thin, crystalline 1D flat phosphorus chains on a Ag(111) substrate.

International audience

A novel technique for the investigation of the radiative contribution to the electromagnetic local density of states is presented. The inelastic tunneling current from a scanning tunneling microscope (STM) is used to locally and electrically excite the plasmonic modes of a triangular gold platelet. The radiative decay of these modes is detected through the transparent substrate in the far field. Emission spectra, which depend on the position of the STM excitation, as well as energy-filtered emission maps for particular spectral windows are acquired using this technique. The STM-nanosource spectroscopy and microscopy results are compared to those obtained from spatially resolved electron energy loss spectroscopy (EELS) maps on similar platelets. While EELS is known to be related to the total projected electromagnetic local density of states, the light emission from the STM-nanosource is shown here to select the radiative contribution. Full electromagnetic calculations are carried out to explain the experimental STM data and provide valuable insight into the radiative nature of the different contributions of the breathing and edge plasmon modes of the nanoparticles. Our results introduce the STM-nanosource as a tool to investigate and engineer light emission at the nanoscale., S.C. acknowledges the financial support of the China Scholarship Council (CSC; No. 201304910386). M.Z.-H. acknowledges the financial support of the European Union under the Project H2020 FETOPEN-2016-2017 “FEMTOTERABYTE” (Project No. 737093) and the “Colombian Administrative Department of Science, Technology and Innovation” - COLCIENCIAS under its “Estancias Postdoctorales 784-2017” call. He also acknowledges the hospitality of the Institute des Sciences Moleculaires d’Orsay and Dr. Nuno de Sousa for his guidance and support using COMSOL Multiphysics. S.M. was supported by the HAPPLE grant (French National Research Agency: ANR-12-BS10-0016). This work is partially funded by the Conseil Regional, Ile-deFrance (DIM Nano-K). J.A. acknowledges Project FIS2016-80174-P of the Spanish MICINN and Project IT1164-19 of the Basque Government.

International audience; Tip-induced oxidation of silicene nano-ribbonsMohamed Rachid Tchalala,*aHanna Enriquez,aAzzedine Bendounan,bAndrew J. Mayne,aG ́erald Dujardin,aAbdelkader Kara,cMustapha Ait Alidand Hamid Oughaddou*aeWe report on the oxidation of self-assembled silicene nanoribbonsgrown on the Ag (110) surface using scanning tunneling microscopyand high-resolution photoemission spectroscopy. The results showthat silicene nanoribbons present a strong resistance towards oxida-tion using molecular oxygen. This can be overcome by increasing theelectricfield in the STM tunnel junction above a threshold of +2.6 V toinduce oxygen dissociation and reaction. The higher reactivity of thesilicene nanoribbons towards atomic oxygen is observed as expected.The HR-PES confirm these observations: even at high exposures ofmolecular oxygen, the Si 2p core-level peaks corresponding to pristinesilicene remain dominant, reflecting a very low reactivity to molecularoxygen. Complete oxidation is obtained following exposure to highdoses of atomic oxygen; the Si 2p core level peak corresponding topristine silicene disappears.

International audience; Silicene, a new 2D material has attracted intense research because of the ubiquitous use of silicon in modern technology. However, producing free‐standing silicene has proved to be a huge challenge. Until now, silicene could be synthesized only on metal surfaces where it naturally forms strong interactions with the metal substrate that modify its electronic properties. Here, the authors report the first experimental evidence of silicene nanoribbons on an insulating NaCl thin film. This work represents a major breakthrough, for the study of the intrinsic properties of silicene, and by extension to other 2D materials that have so far only been grown on metal surfaces.

International audience; We present an experimental investigation of a new polymorphic 2D single layer of phosphorus on Ag(111). The atomically-resolved scanning tunneling microscopy (STM) images show a new 2D material composed of freely-floating phosphorus pentamers organized into a 2D layer, where the pentamers are aligned in close-packed rows. The scanning tunneling spectroscopy (STS) measurements reveal a semiconducting character with a band gap of 1.20 eV. This work presents the formation at low temperature (LT) of a new polymorphic 2D phosphorus layer composed of a floating 2D pentamer structure. The smooth curved terrace edges and a lack of any clear crystallographic orientation with respect to the Ag(111) substrate at room temperature indicates a smooth potential energy surface that is reminiscent of a liquid-like growth phase. This is confirmed by density functional theory (DFT) calculations that find a small energy barrier of only 0.17 eV to surface diffusion of the pentamers (see Supplemental Material). The formation of extended, homogeneous domains is 2 a key ingredient to opening a new avenue to integrate this new 2D material into electronic devices.

The synthesis of blue phosphorene by molecular beam epitaxy (MBE) has recently come under the spotlight due to its potential applications in electronic and optoelectronic devices. However, this synthesis remains a significant challenge. The surface reactivity between the P atoms and the Au atoms should be considered for the P/Au(111) system. In the MBE process, the temperature of the substrate is a key parameter for the growth of blue phosphorene. During the initial growth stage, irregularly shaped Phosphorus clusters grow on top of Au(111) surface at room temperature. When the substrate temperature is increased, these clusters transform into a phosphorene-like structure with a honeycomb lattice. An atom exchange reaction is observed between the P and first layer Au atoms under thermal activation at higher temperature, where the P atoms replace Au atoms to form a blue phosphorene structure within the top Au layer and at the step edges.

In this study, the self-assembled molecular network and electronic properties of Ni-phthalocyanine (NiPc) molecules on monolayer graphene (MLG)/6H-SiC(0001) were studied by room temperature Scanning Tunnelling Microscopy (STM) and Density Functional Theory (DFT) calculations. In this study, a very weak electronic coupling between the graphene and the NiPc molecules is found. This is due to the very small charge transfer of only 0.035e- per molecule. The weak molecule-graphene interaction has two observable consequences: sub-molecular resolution was obtained in the STM spectroscopy at room-temperature with the molecules adsorbed directly on the graphene, and the occupied and unoccupied molecular resonance peaks were observed to shift their position in energy as a function of the tip-surface distance. This is due to the temporary local charging (either positive or negative) that is achieved by decreasing the surface voltage under the STM tip. This may have important consequences for future studies of the opto-electronic properties of such hybrid graphene-molecule systems.

We report results on the growth of an NaCl film on Ag(110) under ultrahigh vacuum conditions. At room temperature, low-energy electron diffraction and scanning tunneling microscopy show that the NaCl film forms a (4×1) superstructure. At RT, the film consists of small-sized islands that coalesce into larger islands at 410 K. These large islands preserve the (4×1) superstructure and cover the entire surface. The apparent heights obtained from the STM images show that the initial thickness of the NaCl islands is one atomic layer, and they present a very small height corrugation. The density functional theory calculations, with and without the inclusion of van der Waals effects, confirm the coexistence of two domains in agreement with the observed structure.

Cet article évoque le cheminement qui a conduit une partie de la communauté scientifique d’Orsay, dès 1960, de la chimie sous rayonnements à la physique des collisions atomiques et moléculaires et à la photophysique, pour aboutir à la création, en 2010, de l’institut des sciences moléculaires d’Orsay, aujourd’hui organisé selon trois grands pôles : la physique moléculaire, les nanosciences, la physique pour la biologie. Si la personnalité des pionniers a joué un rôle essentiel dans l’établissement des concepts, les avancées techniques et les découvertes fortuites ont eu leur importance. This article evokes the pathway which, since 1960, has led a part of the scientific community at Orsay, from radiation chemistry to atomic and molecular collision physics and photophysics, resulting, by intensifying exchanges, in the creation of the Institute for Molecular Sciences at Orsay, presently organized along three main centers of expertise: molecular physics, nanosciences, physics for biology. If the pioneers’ personal qualities have played a major role in the establishment of concepts, technical advances and fortuitous discoveries have made important contributions.

International audience; A new single-image acquisition technique for the determination of the dispersion relation of the propagating modes of a plasmonic multilayer stack is introduced. This technique is based on an electrically-driven, spectrally broad excitation source which is nanoscale in size: the inelastic electron tunnel current between the tip of a scanning tunneling microscope (STM) and the sample. The resulting light from the excited modes of the system is collected in transmission using a microscope objective. The energy-momentum dispersion relation of the excited optical modes is then determined from the angle-resolved optical spectrum of the collected light. Experimental and theoretical results are obtained for metal-insulator-metal (MIM) stacks consisting of a silicon oxide layer (70, 190 or 310 nm thick) between two gold films (each with a thickness of 30 nm). The broadband characterization of hybrid plasmonic-photonic transverse magnetic (TM) modes involved in an avoided crossing is demonstrated and the advantages of this new technique over optical reflectivity measurements are evaluated.

International audience; The long sought-after goal of locally and spectroscopically probing the excitons of two-dimensional (2D) semiconductors is attained using a scanning tunneling microscope (STM). Excitonic luminescence from monolayer molybdenum diselenide (MoSe 2) on a transparent conducting substrate is electrically excited in the tunnel junction of an STM under ambient conditions. By comparing the results with photoluminescence measurements, the emission mechanism is identified as the radiative recombination of bright A excitons. STM-induced luminescence is observed at bias voltages as low as those that correspond to the energy of the optical band gap of MoSe 2. The proposed excitation mechanism is resonance energy transfer from the tunneling current to the excitons in the semiconductor, i.e., through virtual photon coupling. Additional mechanisms (e.g., charge injection) may come into play at bias voltages that are higher than the electronic band gap. Photon emission quantum efficiencies of up to 10 −7 photons per electron are obtained, despite the lack of any participating plasmons. Our results demonstrate a new technique for investigating the excitonic and optoelectronic properties of 2D semiconductors and their heterostructures at the nanometer scale.

International audience; We review the long evolution of DIET (Dynamics at surfaces Induced by Electronic Transitions) that began in the 1960swhenMenzel, Gomer and Redhead proposed their famous stimulated desorption model. DIET entered the “nanoscale” in the 1990s when researchers at Bell Labs and IBMrealized that the Scanning Tunneling Microscope (STM) could be used as an atomic size source of electrons to electronically excite individual atoms andmolecules on surfaces. Resonant and radiant Inelastic Electron Tunneling (IET) using the STM have considerably enlarged the range of applications of DIET. Nowadays, “DIET at the nanoscale” covers a broad range of phenomena at the atomic-scale. This includes molecular dynamics (dissociation, desorption, isomerization, displacement, chemical reactions), vibrational spectroscopy and dynamics, spin spectroscopy and manipulation, luminescence spectroscopy, Raman spectroscopy and plasmonics. Future trends of DIET at the nanoscale offer exciting prospects for new methods to control light and matter at the nanoscale.

International audience

Hydrogen-terminated diamond is known for its unusually high surface conductivity that is ascribed to its negative electron affinity. In the presence of acceptor molecules, electrons are expected to transfer from the surface to the acceptor, resulting in p-type surface conductivity. Here, we present Kelvin probe force microscopy (KPFM) measurements on carbon nanotubes and C60 adsorbed onto a hydrogen-terminated diamond(001) surface. A clear reduction in the Kelvin signal is observed at the position of the carbon nanotubes and C60 molecules as compared with the bare, air-exposed surface. This result can be explained by the high positive electron affinity of carbon nanotubes and C60, resulting in electron transfer from the surface to the adsorbates. When an oxygen-terminated diamond(001) is used instead, no reduction in the Kelvin signal is obtained. While the presence of a charged adsorbate or a difference in work function could induce a change in the KPFM signal, a charge transfer effect of the hydrogen-terminated diamond surface, by the adsorption of the carbon nanotubes and the C60 fullerenes, is consistent with previous theoretical studies.

International audience; We report on the low-energy, electrical generation of light beams in specific directions from planar elliptical microstructures. The emission direction of the beam is determined by the microstructure eccentricity. A very simple, broadband, optical antenna design is used, which consists of a single elliptical slit etched into a gold film. The light beam source is driven by an electrical nanosource of surface plasmon polaritons (SPP) that is located at one focus of the ellipse. In this study, SPPs are generated through inelastic electron tunneling between a gold surface and the tip of a scanning tunneling microscope.

International audience; We report on the performance and inherent artifacts of k-space optical microscopy for the study of periodic arrays of nanoparticles under the various illumination configurations available on an inverted optical microscope. We focus on the origin of these artifacts and the ways to overcome or even benefit from them. In particular, a recently reported artifact, called the " condenser effect, " is demonstrated here in a new way. The consequences of this artifact (which is due to spurious reflections in the objective) on Fourier-space imaging and spectroscopic measurements are analyzed in detail. The advantages of using k-space optical microscopy to determine the optical band structure of plasmonic arrays and to perform surface plasmon resonance experiments are demonstrated. Potential applications of k-space imaging for the accurate lateral and axial positioning of the sample in optical microscopy are investigated.

International audience; We report on the optical and electrical excitation of the modes of a “hybrid” waveguide consisting of a single organic nanofiber on a thin gold film. In the first set of experiments, light is used to excite the photoluminescence of an organic nanofiber on a thin gold film and the resultingemission is analyzed using Fourier-space leakage radiation microscopy. Two guided modes and the dispersion relations of this hybrid waveguide are thus determined. From numerical calculations, both a fundamental and excited mode of mixed photonic−plasmonic character are identified. In a second experiment, a local electrical nanosource of surface plasmon polaritons (SPPs) is coupled to the hybrid waveguide. The SPP nanosource consists of the inelastic electron tunnel current between the tip of a scanning tunneling microscope (STM) and the gold film. We show that the electrically excited SPPs couple to the fundamental mode and that the coupling efficiency is highest when the SPP nanosource is aligned with the nanofiber axis. Moreover, the electrically excited SPPs strongly scatter into out-of-plane light at the nanofiber end. This light from scattered SPPs measured in the substrate is phase shifted by about π with respect to the direct light emission from beneath the STM tip. These experiments lead to a better understanding of the processes that must be optimized in order to exploit such hybridwaveguide structures.

Silicene is emerging as a two-dimensional material with very attractive electronic properties for a wide range of applications; it is a particularly promising material for nano-electronics in silicon-based technology. Over the last decade, the existence and stability of silicene has been the subject of much debate. Theoretical studies were the first to predict a puckered honeycomb structure with electronic properties resembling those of graphene. Though these studies were for free-standing silicene, experimental fabrication of silicene has been achieved so far only through epitaxial growth on crystalline surfaces. Indeed, it was only in 2010 that researchers presented the first experimental evidence of the formation of silicene on Ag(1 1 0) and Ag(1 1 1), which has launched silicene in a similar way to graphene. This very active field has naturally led to the recent growth of silicene on Ir(1 1 1), ZrB2(0 0 0 1) and Au(1 1 0) substrates. However, the electronic properties of epitaxially grown silicene on metal surfaces are influenced by the strong silicene–metal interactions. This has prompted experimental studies of the growth of multi-layer silicene, though the nature of its “silicene” structure remains questionable. Of course, like graphene, synthesizing free-standing silicene represents the ultimate challenge. A first step towards this has been reported recently through chemical exfoliation from calcium disilicide (CaSi2). In this review, we discuss the experimental and theoretical studies of silicene performed to date. Special attention is given to different experimental studies of the electronic properties of silicene on metal substrates. New avenues for the growth of silicene on other substrates with different chemical characteristics are presented along with foreseeable applications such as nano-devices and novel batteries.

International audience; Gold nanoparticles are used extensively in many aspects of science. In particular, their optical properties are exploited in diverse areas such as biosensing1, molecular rulers2, organic solar cells3 and photocatalysis,4 to name a few. In all these applications, light is used to excite the surface plasmon resonance and/or intra/interband transition in the gold nanoparticles5,6. However, the ability to use electrons rather than photons to induce light from gold nanoparticles opens up horizons which would otherwise be unattainable.Two main methods may be considered practically for obtaining light from the electrical excitation of gold nanoparticles: one can use low energy (~ 2 eV) tunnel electrons from a scanning tunneling microscope7,8 (STM) or high energy (~ 30 keV) electrons from a scanning electron microscope9,10. In both cases, the main advantage is the small size of the excitation when using electrons as compared to photons. Whereas the size of a photonic probe is limited by diffraction to a few hundred nanometers, the size of an electronic probe is truly nanoscale11,12. As will be shown in the following, such extremely high spatial selectivity enables completely new insights into the optical properties of gold nanoparticles. Indeed, excitation with a spatial selectivity on the order of 10 nm may not only be used to investigate single gold nanoparticles but moreover to provoke the emission of light by exciting specific locations inside a single gold nano-object. Low energy electrical excitation of a gold nanoparticle is of further interest for applications such as electron/photon transduction at the nanoscale. Using this technique, an electrical signal may be converted into a photonic one. This will lead to new applications in future optoelectronic devices.From an historical point of view, electron-to-photon energy conversion has played an emblematic role in physics over the last 150 years. The invention of the Crook’s tube in 1879, where the energy of an electrical discharge is converted into light, has led to a number of discoveries; the X-rays by W. Röntgen in 1895, the fluorescent tube by T. Edison in 1895, the discovery of the electron by J.J. Thomson in 1897…. More recently, the conversion of energy from high energy (keV) electrons to light via metallic films was first predicted by R.A. Ferrell13 in 1958 and experimentally observed by Steinmann14 and Brown et. al15 in 1960. Concerning low energy (~ 2 eV) tunnel electrons, the first observation of energy conversion into light dates back to 1976 when J. Lambe and S.L. McCarthy16 discovered this new method for the generation of light. This light emission method was extended to tunnel electrons from the STM in 1988 by J.K. Gimzewski et al.17,18 a few years after the invention of the STM by G. Binnig and H. Rohrer19.In this chapter, we review recent studies of electron-to-photon energy conversion in gold nanoparticles. In Section 1.2 we describe the fundamental mechanisms for the electrical generation of light from gold nanoparticles when using low energy (~ 2 eV) tunnel electrons from a scanning tunneling microscope or high energy (~ 30 keV) electrons from a scanning electron microscope. In Section 1.3 we report on examples of such electron/photon transduction experiments in various gold nanostructures.

International audience; Plasmonic lenses, even of simple design, may have intricate spectral behavior. The spectral response of a plasmonic lens to a local, broadband excitation has rarely been studied despite its central importance in future applications. Here we use the unique combination of scanning tunneling microscopy (STM) and angle-resolved optical spectroscopy to probe the spectral response of a plasmonic lens. Such a lens consists of a series of concentric circular slits etched in a thick gold film. Spectrally broad, circular surface plasmon polariton (SPP) waves are electrically launched from the STM tip at the plasmonic lens center, and these waves scatter at the slits into a narrow, out-of-plane, light beam. We show that the angular distribution of the emitted light results from the interplay of the size of the plasmonic lens and the spectral width of the SPP nanosource. We then propose simple design rules for optimized light beaming with the smallest possible footprint. The spectral distribution of the emitted light depends not only on the SPP nanosource, but on the local density of electromagnetic states (EM-LDOS) at the nanosource position, which in turn depends on the cavity modes of the plasmonic microstructure. The key parameters for tailoring the spectral response of the plasmonic lens are the period of the slits forming the lens, the number of slits, and the lens inner diameter.

International audience; We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52–60 eV and 3 keV is studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using scanning tunneling microscopy (STM). We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on NaCl thick films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges.

International audience; We report on the optical and electrical excitation of the modes of a “hybrid” waveguide consisting of a single organic nanofiber on a thin gold film. In the first set of experiments, light is used to excite the photoluminescence of an organic nanofiber on a thin gold film and the resulting emission is analyzed using Fourier-space leakage radiation microscopy. Two guided modes and the dispersion relations of this hybrid waveguide are thus determined. From numerical calculations, both a fundamental and excited mode of mixed photonic–plasmonic character are identified. In a second experiment, a local electrical nanosource of surface plasmon polaritons (SPPs) is coupled to the hybrid waveguide. The SPP nanosource consists of the inelastic electron tunnel current between the tip of a scanning tunneling microscope (STM) and the gold film. We show that the electrically excited SPPs couple to the fundamental mode and that the coupling efficiency is highest when the SPP nanosource is aligned with the nanofiber axis. Moreover, the electrically excited SPPs strongly scatter into out-of-plane light at the nanofiber end. This light from scattered SPPs measured in the substrate is phase shifted by about π with respect to the direct light emission from beneath the STM tip. These experiments lead to a better understanding of the processes that must be optimized in order to exploit such hybrid waveguide structures.

Using scanning tunneling microscopy, low energy electron diffraction measurements, and ab initio calculations based on density functional theory, we present atomic models of the ( 13 × 13 )R13.9° silicene superstructure grown on Ag(1 1 1). The STM images reveal two co-existing atomic arrangements with two different orientations of the silicene sheet relative to the Ag(1 1 1) surface. DFT calculations with and without the inclusion of van der Waals interactions show corrugated Si atomic positions for both orientations implying a significant interaction with Ag(1 1 1) surface. The electronic structure of both silicene and Ag(1 1 1) surface are sufficiently affected that new interface states emerge close to the Fermi level.

Une source de lumiere de dimension atomique est realisee a l’aide d’un microscope a effet tunnel (STM). Cette source de photons est localisee a la jonction entre une pointe, que l’on peut deplacer avec une precision atomique, et un echantillon metallique. Elle se distingue par une excitation de nature electronique (le courant tunnel), et fait egalement intervenir des plasmons de surface presents dans la jonction tunnel. La jonction tunnel est ainsi une « source electrique » de plasmons de surface. Cependant, on est encore loin de comprendre le fonctionnement de cette source optique et plasmonique et d’en avoir exploite toutes les possibilites.

The remarkable properties of graphene stem from its two-dimensional (2D) structure, with a linear dispersion of the electronic states at the corners of the Brillouin zone (BZ) forming a Dirac cone. Since then, other 2D materials have been suggested based on boron, silicon, germanium, phosphorus, tin, and metal di-chalcogenides. Here, we present an experimental investigation of a single silicon layer on Au(111) using low energy electron diffraction (LEED), high resolution angle-resolved photoemission spectroscopy (HR-ARPES), and scanning tunneling microscopy (STM). The HR-ARPES data show compelling evidence that the silicon based 2D overlayer is responsible for the observed linear dispersed feature in the valence band, with a Fermi velocity of "Equation missing" comparable to that of graphene. The STM images show extended and homogeneous domains, offering a viable route to the fabrication of silicene-based opto-electronic devices.

International audience; Surface plasmon polariton (SPP) beams with an in-plane angular spread of 8 degrees are produced by electrically exciting a 2D plasmonic crystal using a scanning tunneling microscope (STM). The plasmonic crystal consists of a gold nanoparticle (NP) array on a thin gold film on a glass substrate and it is the inelastic tunnel electrons (IET) from the STM that provide a localized and spectrally broadband SPP source. Surface waves on the gold film are shown to be essential for the coupling of the local, electrical excitation to the extended NP array, thus leading to the creation of SPP beams. A simple model of the scattering of SPPs by the array is used to explain the origin and direction of the generated SPP beams under certain conditions. In order to take into account the broadband spectrum of the source, calculations realized using finite-difference time-domain (FDTD) methods are obtained, showing that bandgaps for SPP propagation exist for certain wavelengths and indicating how changing the pitch of the NP array may enhance the SPP beaming effect. (C) 2016 Optical Society of America

In the framework of density functional theory, the adsorption of the halogenated polycyclic aromatic hydrocarbon 2,11-diiodohexabenzocoronene (HBC-I2 ) on the SiC(0001) 3×3 surface has been investigated. Nondissociative and dissociative molecular adsorption is considered, and simulated scanning tunneling microscopy (STM) images are compared with the corresponding experimental observations. Calculations show that dissociative adsorption is favorable and reveal the crucial importance of the extended flat carbon core on molecule-surface interactions in dissociative adsorption; the iodine atom-surface interaction is of minor importance. Indeed, removing iodine atoms does not significantly affect the STM images of the central part of the molecule. This study shows that the dissociation of large halogenated polycyclic aromatic hydrocarbon molecules can occur on the SiC surface. This opens up interesting perspectives in the chemical reactivity and functionalization of wide band gap semiconductors.

International audience; A local, low-energy, electrical method for the excitation of localized and propagating surface plasmon polaritons (SPPs) is attractive for both fundamental and applied research. In particular, such a method produces no excitation background light and may be integrated with nanoelectronics. Here we report on the electrical excitation of SPPs through the inelastic tunneling of low-energy electrons from the tip of a scanning tunneling microscope (STM) to the surface of a two-dimensional plasmonic lens. The plasmonic structure is a series of concentric circular slits etched in a thick gold film on a glass substrate. An outgoing circular SPP wave is generated from the tip-sample junction and is scattered into light by the slits. We compare the resulting emission pattern to that observed when exciting SPPs on a thin, unstructured gold film. For optimized parameters, the light emitted from the plasmonic lens is radially polarized. We describe the effects of the slit period and number, and lens diameter on the emission pattern and we diskuss how the light beam of low divergence is formed.

Over the last decade, the existence and stability of silicene has been the subject of numerous studies. Indeed, silicene resembles graphene as it is a two-dimensional material arranged in a honeycomb lattice. Electronically, the main difference between carbon and silicon is the strong preference for sp3 over sp2 in silicon. It was only in 2010 that researchers presented the first experimental evidence of the formation of silicene on Ag(110) and Ag(111), which has launched a rush for silicene in a similar way as for graphene. This very active field has naturally led to the recent growth of silicene on other substrates such as Ir, ZrB2 and Au. However, unlike graphene, the existence of silicene as a stand-alone material remains elusive. We present in this chapter the state of the art of silicene growth on Ag(111) and Au(110) substrates.

International audience; The inelastic tunnel current in the junction formed between the tip of a scanning tunneling microscope (STM) and the sample can electrically generate optical signals. This phenomenon is potentially of great importance for nano-optoelectronic devices. In practice, however, the properties of the emitted light are difficult to control because of the strong influence of the STM tip. In this work, we show both theoretically and experimentally that the sought-after, well-controlled emission of light from an STM tunnel junction may be achieved using a nonplasmonic STM tip and a plasmonic nanoparticle on a transparent substrate. We demonstrate that the native plasmon modes of the nanoparticle may be used to engineer the light emitted in the substrate. Both the angular distribution and intensity of the emitted light may be varied in a predictable way by choosing the excitation position of the STM tip on the particle.

Controlling the intrinsic optical and electronic properties of a single molecule adsorbed on a surface requires electronic decoupling of some molecular orbitals from the surface states. Scanning tunneling microscopy experiments and density functional theory calculations are used to study a perylene molecule derivative (DHH-PTCDI), adsorbed on the clean 3 × 3 reconstructed wide band gap silicon carbide surface (SiC(0001)-3 × 3). We find that the LUMO of the adsorbed molecule is invisible in I(V) spectra due to the absence of any surface or bulk states and that the HOMO has a very low saturation current in I(z) spectra. These results present a paradox that the molecular orbitals are electronically isolated from the surface of the wide band gap semiconductor even though strong chemical bonds are formed.

Phosphorene is a new two-dimensional material composed of a single or few atomic layers of black phosphorus. Phosphorene has both an intrinsic tunable direct band gap and high carrier mobility values, which make it suitable for a large variety of optical and electronic devices. However, the synthesis of single-layer phosphorene is a major challenge. The standard procedure to obtain phosphorene is by exfoliation. More recently, the epitaxial growth of single-layer phosphorene on Au(111) has been investigated by molecular beam epitaxy and the obtained structure has been described as a blue-phosphorene sheet. In the present study, large areas of high-quality monolayer phosphorene, with a band gap value at least equal to 0.8 eV, have been synthesized on Au(111). Our experimental investigations, coupled with DFT calculations, give evidence of two distinct phases of blue phosphorene on Au(111), instead of one as previously reported, and their atomic structures have been determined., Comment: This paper reports on the epitaxial synthesis of blue phosphorene

Photoinduced trans-cis isomerization studies of stilbene molecules in the gas phase have led to a precise understanding of the corresponding molecular dynamics. Yet, when such molecules are adsorbed on surfaces, these reactions are expected to be strongly modified as compared to what is know in the gas phase. In this work, a low temperature (5 K) scanning tunneling microscope (STM) is used to image the trans-stilbene molecules deposited on a Si(100)-2 x 1 surface at 12 K. trans-Stilbene undergoes conformational changes during the adsorption process such that four different stilbene conformers are observed: trans-stilbene (TS), cis-stilbene (CS), and two new conformers I(1) and I(2). Furthermore, electronic excitation of individual stilbene molecules, by means of tunnel electrons, is shown to activate specific reversible molecular surface isomerization (TS I(1) and CS I(2)) combined with diffusion across the surface. Calculated STM topographies, using the tight binding method, indicate that the CS and TS molecules are physisorbed. The molecular conformations of the surface isomers I(1) and I(2) are suggested to be analogous to transient states conformations of the stilbene molecule when stabilized by the silicon surface. The measurements of the molecular surface isomerization and diffusion reaction yields are used to build a qualitative potential energy surface of the various stilbene reactions. The molecular surface-isomerization dynamics is shown to be influenced by the type of dopant (n or p). This is related to surface charging, which reveals modifications in the stilbene ionization potential.

Decoupling the electronic properties of a molecule from a substrate is of crucial importance for the development of single-molecule electronics. This is achieved here by adsorbing pentacene molecules at low temperature on a hydrogenated Si(100) surface (12 K). The low temperature (5 K) scanning tunneling microscope (STM) topography of the single pentacene molecule at the energy of the highest occupied molecular orbital (HOMO) tunnel resonance clearly resembles the native HOMO of the free molecule. The negligible electronic coupling between the molecule and the substrate is confirmed by theoretical STM topography and diffusion barrier energy calculations.

The adsorption of the cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE) molecule on the clean Si(100)-2 × 1 surface was studied using scanning tunneling microscopy at room temperature. These molecules belong to the class of photochromic dye molecules that undergo reversible photoisomerization between the open and closed isomers when irradiated with light. This is the first observation of individual photochromic molecules on a surface where it is possible to identify the two photoisomers. By changing the ambient lighting conditions prior to deposition, the relative proportion of the two forms of molecules observed on the silicon surface can be changed. The deduction is that the triangular form corresponds to the closed isomer, and the three-lobed form corresponds to the open isomer.

International audience; We study the scattering of electrically excited surface plasmon polaritons (SPP) from individual nanostructures. The tunneling electrons from a scanning tunneling microscope (STM) are used to excite an outgoing , circular SPP wave on a thin (50-nm) gold film on which isolated gold nanoparticles (NPs) have been deposited. Interaction of the excited SPPs with the NPs leads to both in-plane (SPP-to-SPP) and out-of-plane (SPP-to-photon) scattering. We use SPP leakage radiation microscopy to monitor the interference between the incident and in-plane scattered SPP waves in the image plane. By changing the location of the STM tip, the distance of the pointlike SPP source to the scatterers can be varied at will, which constitutes a key advantage over other existing techniques. As well, the out-of-plane scattered radiation interferes with the direct light emission from the STM tip in the back focal plane (Fourier plane). This confirms the mutual coherence of the light and SPP emission resulting from the inelastic tunneling of an electron in the STM junction. We use this effect to demonstrate that SPP-to-photon scattering at NPs is highly directional.

Electrical nanosources of surface plasmons will be an integral part of any future plasmonic circuits. Three different types of such nanosources (based on inelastic electron tunneling, high energy electron bombardment, and the electrical injection of a semiconductor device) are briefly described here. An example of a fundamental experiment using an electrical nanosource consisting of the tunnel junction formed between a scanning tunneling microscope (STM) and a metallic sample is given. In this experiment, the temporal coherence of the broadband STM-plasmon source is probed using a variant of Young's double slit experiment, and the coherence time of the broadband source is estimated to be about 5-10 fs.

Powering and controlling the operation of a single molecule adsorbed on a surface can be achieved by using the tip of a scanning tunnelling microscope (STM) as an atomic-size source of electrons. We review the various electronic excitation processes induced by the electrons from the STM tip which are able to activate the functions of a molecular nanomachine. In particular, we review recent results illustrating the electronic control of molecular dynamics at the level of a single molecule.

Ion photostimulated desorption (PSD) is a specific surface sensitive process. This paper reviews ion PSD following core level excitation of molecules adsorbed on the Si(111) 7 × 7 and Si(100) 2 × 1 surfaces. Several aspects of ion PSD will be discussed; (i) the use of ion PSD as a tool for investigating the adsorption configurations of O2 on Si(111) 7 × 7, (ii) the relevance of ion PSD for probing the physisorption–chemisorption transition of benzene molecules on Si(111), (iii) the comparison between ion PSD of methanol adsorbed on Si(111) and photofragmentation of the methanol molecule in the gas phase, (iv) the comparison between ion PSD of formic acid adsorbed on the Si(111) and Si(100) surfaces and (v) the chemical selectivity of ion PSD following core level excitation of NO dissociatively adsorbed on Si(111). Finally, the interplay between ion dynamics and electronic relaxation following core level excitation and Auger processes will be discussed in the case of O+ photodesorption from O2 adsorbed on Si(111) 7 × 7, as investigated via isotope and temperature effects on both the intensities and kinetic energies of the desorbed ions.

The adsorption of hydrogen on semiconductors strongly modifies the electronic and chemical properties of the surfaces, whether on the surface or in the sub-surface region. This has been the starting point, in recent years, of many new areas of research and technology. This paper will discuss the properties, at the atomic scale, of hydrogenated semiconductor surfaces studied with scanning tunnelling microscopy (STM) and synchrotron radiation. Four semiconductor surfaces will be described – germanium(1 1 1), silicon(1 0 0), silicon carbide(1 0 0) and diamond(1 0 0). Each surface has its particularities in terms of the physical and electronic structure and in regard to the adsorption of hydrogen. The manipulation of hydrogen on these surfaces by electronic excitation using electrons from the STM tip will be discussed in detail highlighting the excitation mechanisms. The reactivity of these surfaces towards various molecules and semiconductor nanocrystals will be illustrated.